1. Field of the Invention
This invention relates to a magnetic recording medium, and more particularly to a high-output ferromagnetic metal thin-film recording medium which has an improved resistance to corrosion.
2. Description of the Prior Art
There has been in wide use so-called coated magnetic recording media comprising a non-magnetic base and a magnetic layer which is formed by coating magnetic powder together with binder on the non-magnetic base and drying the same.
However, recently, there has been an increasing demand for high-density recording, and the demand cannot be satisfied by the conventional coated magnetic recording media.
Accordingly, there has been recently developed and put into practical use a so-called metal thin-film magnetic recording medium having a ferromagnetic metal thin-film formed on a non-magnetic base by a vacuum film-forming method such as vacuum deposition, sputtering or the like.
The metal thin-film magnetic recording medium has excellent properties as a magnetic recording medium for the high-density recording. For example, it is excellent in electromagnetic conversion properties in short wavelength recording due to its large coercive force and squareness ratio, and in the metal thin-film magnetic recording medium, the thickness loss is very small due to its very thin magnetic layer.
Among the metal thin-film magnetic recording medium, those having a magnetic layer of Co alloys exhibit excellent magnetic recording properties. Especially those obtained by oblique deposition of Co.sub.80 Ni.sub.20 alloy has been known as "deposition tape" and has been in wide use.
Further, a metal thin-film magnetic recording medium having a Co--CoO magnetic thin-film provided with vertical magnetic anisotropy is considered to be promising for high-density recording.
However, since Co does not have sufficient resistance to corrosion, the metal thin-film magnetic recording medium having a magnetic layer of Co alloys can easily rust when the thin-film sweats or is placed in an atmosphere containing therein corrosive gas, whereby the magnetic properties deteriorate or the magnetic head can clog during running. Various efforts to overcome the problem by adding Ni, Cr or oxides to the metal thin-film have been made. However, at present, such efforts have not succeeded. Further, addition of additives involves deterioration of the magnetic properties and accordingly it has been difficult to improve the resistance to corrosion by addition of additives without adversely affecting the electromagnetic conversion properties. For example, in the early deposition tape, the thin-film can rust by merely letting it stand in the air or the magnetic flux can reduce by half in one year or so. Accordingly, the early deposition tape is disadvantageous from the viewpoint of storage of information.
Further it has been proposed to limit the proportion of oxygen atoms to the whole atoms in the magnetic layer and cover the magnetic layer or prismatic grains forming the magnetic layer with an oxide layer, thereby improving the resistance to corrosion of the thin-film. (See, for instance, Japanese Unexamined Patent Publication Nos. 56(1981)-15014 and 57(1982)-179951, and the like.
Further, as disclosed in Japanese Unexamined Patent Publication No. 62(1987)-275316, the resistance to corrosion of the metal thin-film magnetic recording medium can be improved by forming a passive film of amorphous cobalt compound on the surface of the ferromagnetic metal thin-film. Further a method of forming a passive film of Co.sub.3 O.sub.4 on the ferromagnetic metal thin-film is disclosed in Japanese Unexamined Patent Publication No. 60(1985)-160027.
However, in accordance with the prior arts described above, the resistance to corrosion of the metal thin-film magnetic recording medium cannot be satisfactorily improved without increasing the thickness of the non-magnetic layer. However increase in the thickness of the non-magnetic layer increases the spacing loss and deteriorates the electromagnetic conversion properties.
In "Japanese Applied Magnetics Academy Transactions" Vol. 14, No. 2, 1990, it is reported that corrosion of the thin-film progresses by reaction of CoO and Co(OH).sub.2 with oxygen to form Co(OH).sub.2 and CoOOH. However, no attempt to control the amount of Co(OH).sub.2 and CoOOH has been made.